Power transmission



May 22, 1956 Filed Nov. 25 1951 E. F. KLESSIG EIAL 2,746,392

POWER TRANSMISSION 3 Sheets-Sheet 1 AT TOR NEY y 1956 E; F. KLESSIG EPOWER TRANSMISSION 3 Sheets-Sheet 2 I Filed Nov. 23. 1951 FIG.3

- INVENTOR5 ERNST F KLESSIG GLENN M JONES ATTORNEY May 22, 1956 E. F.KLESSIG m-AL 2,746,392

POWER TRANSMISSION Filed Nov. 23, 1951 s Sheets-Sheet 3 ll INVENTORS J,'56 ERNST F KLESSIG 9 GLENN M. JONES gmm ATTORNEY 2,746,392 rowanTRANSMISSION Emir. Klessig, Berkley, and- Glenn M. Jones, Farmington,Mich assignors to Vickers Incorporated, Detroit, Mich., a corporation ofMichigan Application November 23, 1951, Serial No. 257,908 Claims. (Cl.10342) particularly to pumping units of a type well suit'edfor use inthe mobile equipment field comprisingpumping mechanism combined with a.valve or valves integrated into a compact unit. These units have had awide acceptance in the automotive and agricultural fields due to theircompactness and ease of installation. Simplicity and ruggedness ofconstruction, low cost, long life, and efiicient operation are ofparamount importancein the design of such a. unit. With the increasingemphasis being put on passenger car applications of hydraulic power,such as hydraulic steering boosters, hydraulic window lifts, etc.,quietness of operation has become an additionally important criterion ofsuch units.

When regarded as a prime mover for accessory drives, the engine. of amotor vehicle leaves much to be desired since. the operating speed mayvary from perhaps 400 R.. P. M. to 4000 R. P. M. and the output of,.forexample, a fixed displacement pump driven thereby will vary inthe sameratio. The problem is not one to be solved by merely introducing gearingbetween the engine and the hydraulic pump since the 10:1 ratio .betweenhigh and low speed still persists and, as in the case of a steeringbooster, output requirements of the pump may be as high, or higher,while the engine idles than while itis at top speed. Further, thecontrol valve used in many steeringboosters is an open center type inwhich machining. tolerances are veryclosely controlled. .For example,valve land widths may be held to axial. thickness tolerances of theorder of onesthousanclth, of an inch. Such. painstaking and expensiveconstruction is utilized togive the vehicle operator a nicety andpreciseness of feel in steering. which. would be largely lost if the oilflow through the valve were allowed too much variance. A variabledisplacement pump is of course a possible solution. to the problem butits cost and complexity often make it an unacceptable solution.

It, is. an object of the present invention to provide an improved lowcost and compact hydraulic power unit which is exceptionally welladapted for use with a variahle speed prime mover such as the. engine ofa motor vehicle. v t

More particularly, it is an object to provide such a unit, utilizing afixed displacement pump, in which the output tov the driven device ismaintained. at a rclatively constant rate regardless of speed variationsof theprime mover. p

A further major problem encountered in automotive applications is thephenomenon known in the art as cavitation, which normallyoccurs at highpump speeds. It is well known in the art that caviation and itsattendant noise and wear can be eliminated by maintaining sufiicientpressure on the pump inlet to prevent the creation 2,746,392 PatentedMay 22 1956 2 ofivoids in the working fluid. For relatively low speedoperation it may be sufficient to merely place the pump inlet in fluidcommunication with a reservoir which is at, or even below, the level ofthe pump inlet, and atmospheric pressure on the fluid in the tank willsuflice to main- 7 tain the pump inlet full of oil thus preventingcavitation. Fluid returning from the motor will be directed into thereservoir from which the pump is supplied. This type of circuit is knownas an open circuit and hasman'y iiiherent advantages among which areautomatic replenishment of leakage losses, continual exchange of. theworking fluid for cool, clean deaerated fluid from the reservoir, andeasy and complete initial filling of the system by merely putting fluidinto the reservoir.

However, as the pump speed increases cavitation may result due to theinadequancy of atmospheric pressure for keeping the pump inlet passagesfull. Elevation of the reservoir to a height sufficient to increase theliquid head enough to prevent cavitation is, of course, one solution,but in most cases is impractical and in many cases is impossible. Amodification of the open circuit by the insertion of a second pump,which may be called a. supercharge pump, in the line between thereservoifand the working pump inlet to provide suflicient pressure onthe working pump inlet is another solution of the problem and permitsretention of the advantages of the open circuit. This arrangement hasthe very substantial disadvantage of requiring an additional pump of acapacity which must exceed that of the working pump.

To avoid the disadvantage of the large supercharge pump required for usewith an 'open circuit, the system known as a closed circuit is in wideuse. The distinguishing characteristic of the closed circuit is that.substantially all the fluid delivered by the working pump to the motorreturns directly to the pump inlet without passingv through thereservoir." A typical closed circuit includes 31 working pump and amotor having two lines therebetwe'en, one for delivery and one forreturn, a reservoir, and a supercharge pump having its inlet connectedto the reservoir and its outlet connected into the return line betweenthe working pump and the motor. The. supercharge pump maintains anadequate pressure on the working pump inlet to prevent cavitation. Theadvantage of this system is that a smaller supercharge pump is required"than is,

necessary in the modified open system. Disadvantages of the. closedsystem include the requirement of asecond pump, however small, and thatthe fluid returning to the working pump from the motor does not receivethe benefits derived from a period of relative quiescence in thereservoir as in the open circuit.

Another method of producing a pressure on the pump inlet sufficient toprevent cavitation has been the applica tion of Bernoullis principlewherein the kinetic energy possessed by rapidly moving oil is convertedin part to a static head on the pump inlet passages. This has beenaccomplished. in the past by inserting a venturi in the return linebetween the pump and the motor in a closed circuit. The advantage ofthis method of producing pressure is that. no second pump is required.Such a device may be seen in the patent toDavis 2,251,664. 7

It can be seen from the toregomg that a system wh ch would provide theadvantages of open circuit operation at speeds below that at whichcavitation occurs and which, asthe pump speed increases beyond thatpoint, will increase the pressure on the pump inlet so as to preventcavitation is highly desirable. The above-mentioned patent to Davisshows such a system but employs sliding valve means, responsive to flowin the return line, used solely for the purpose of providing opencircuit operation at low speed and an automatic change to closed circuitoperation, thus increasing the pump inlet pressure at high speed. Y

' 'It is'an object'of the present invention to provide a system which,while retaining the desirable characteristics of open circuit operationat'low speeds, will automatically operate as a closed circuit as thepump speed approaches the point where cavitation occurs, and increasethe pressure on the pump inlet zone so as to eliminate cavitation.Further, it is an object to accomplish this change in circuit operationwithout the use of any special valve or moving part or auxiliary pump.

Other problems encountered in such applications are the necessity forreplenishing to compensate for leakage 'losses, removal of air entrainedin the operating fluid and cooling the fluid in the system.

Another object is to provide such a unit in which leakage losses arecontinually made up from the reservoir and in which, during closedcircuit operation, a portion of the working fluid is bled into thereservoir and replaced by fresh, cool, clean, and deaerated fluid fromthe reser- A still further object is to provide such a unit at a lowcost but without sacrifice of operating efliciency. This has beenaccomplished by utilizing basic pumping structure well known in the artin the novel combination described herein.

Further objects and advantages of the present invention will be apparentfrom the following description, ireference being had to the accompanyingdrawings wherein a preferred form of the present invention is clearlyshown.

In the drawings: Figure 1 is a sectional view of a preferred form of thepresent invention taken on line 1-1 of Figure 2.

Figure 2 is an end elevation, partially in section, of the unit shown inFigure 1.

Figure 3 is a section on line 33 of Figure 1. Figure 4 is a section online 4-4 of Figure 1.

Figure 5 is a section on line S-5 of Figure 1.

Figure 6 is a section on line 6-6 of Figure 1.

Figure 7 is a section on line 77 of Figure 3. Figure 8 is a view on line88 of Figure 1.

Figure '9 is a schematic diagram of the unit, shown in atypicalapplication.

Referring to Figure 1, there is shown a pump generally designated 10which is of the well-known radially sliding vane type and includes ahousing 14 composed of three sections arranged in a sandwich relation.These three I A drive shaft 34 extends from the body portion 16 and :issupported therein by bearings 36 and 38. A shaft seal 40encircles theshaft 34 in the usual manner. inner end of the shaft 34 carries a splineat 42 which eni gages the rotor 44 of the pumping mechanism. Rotor 44.is mounted between parallel plane surfaces comprising the 'face 46 ofthe body portion 16 and the face 48 of a floatably mounted pressureplate 50. Rotor 44 is encircled by 'a generally elliptical cam contour52 machined in the ring portion 18. Radial slots 54 in rotor 44 containradially slidable vanes 56. The outward ends of vanes 56 are "maintainedagainst 'the cam contour 52 by centrifugal .force aided by fluidpressure in channels 58 of the rotor 44 jjwhich communicate withenlarged portions 60 of the inner .ends of the van'e slots 54. Rotor 44,vanes 56, cam contour 52 and surfaces 46 and 48 define pairs ofdiametricalf. ly opposed high pressure, ,or pumping, zones 62 and lowThe pressure, or suction, zones 64.

Pressure plate 50 is mounted in a'bore 66 in the head "portion 20 of thehousing 14 and coacts therewith to form a pressure chamber 68. A spring7 0 resiliently biases pressure plate 50 into fluidsealing engagementwith rotor 44.

The requisite porting to the inlet zones 64 is supplied by a pair ofkidney shaped ports 72 and 74 in the face 46 of the body portion 16.Porting from the pumping zones 62 is supplied by a pair of kidney-shapedports 76 and 78 extending completely through the pressure plate 50 intothe pressure chamber 68. Body portion 16, ring portion 18, and pressureplate 50 are maintained in the proper angular relation by. dowel pinsextending from body portion 16 through ring portion 18 and into pressureplate 50. It is apparent that the working pressure of the pump willexist in pressure chamber 68 and be exerted on pressure plate 50 so asto aid the spring 70 with a force proportional to the pump workingpressure.

The manner in which kidney-shaped ports 72 and 74 overlie suction zones64 and kidney ports 76 and 78 overlie pumping zones 62 can best be seenby reference to Figures 4 and 6. Thedotted outline of cam contour 52indicates the actual positional relation between the cam ring and thekidney ports. A number of drilled holes 82 in the pressure plate 50provide fluid communication between pressure chamber 68 and the channels58 in the rotor 44 for the purpose previously mentioned. A pair ofcrossover passages 84 and 86 in ring section 18 aflord communicationbetween the kidney ports 72 and 74, respectively, and one of each of twopairs of blind holes 88 and 90 in the pressure plate 50. Thisarrangement permits the suction zones ,64 to receive fluid from bothsides of the rotor, thus producing a more satisfactory inlet condition.

Kidney ports 72 and 74 are at the termini of a branched passage 92, inthe housing portion 16, which is in fluid communication with a passage94 leading to the exterior of body portion 16 and emerging therefrom ina flange 96. The face 98 of flange 96 is coplanar with the face 100 of asimilar flange 102 on the head portion 20 of housing 14. A stepped bore104 extends from flange 102 to the interior of head portion 20 andincludes an enlarged portion 106 and a relatively constricted portion108.

Bore 104 is intersected by three other bores in head portion 20. Thefirst of these, bore 110, extends from the enlarged portion 106 of bore104 to the exterior of the head portion 20 of the housing 14 and isequipped at its outer end with a threaded connection port 112.

The second of these intersecting bores, designated 114, breaks into thebore 104 in its constricted portion 108.

'Bore 114 extends transversely into head portion 20 from a boss 117.This can best be seen by reference to Figure 7. Bore 114 includes aspring chamber 115 and a relatively reduced portion 116. A valve seat118 is formed at the juncture of the smaller and larger portions of'thebore. A relief valve 120 is inserted in the bore and is resilientlybiased-against the valve seat 118 by a spring 121. Spring 121 isretained in the bore by means of a fluid sealing plug 122 which closesthe outer end of bore 114. Spring guide 124 serves to limit the travelof valve 120 away from the seat 118 by making contact with the plug 122.Contact with seat 118 of course determines the normal position of thevalve 120. Valve 120 extends inward considerablybeyond seat 118 and theextension includes a necked down'portion 126 and a pilot portion 128.Pilot portion 128 serves the dual function of maintaining the valve inalignment with the seat and providing a dashpot action. The dashpotaction is obtained by maintaining a relatively close fit between thepilot portion 128 of the valve 120 and the reduced portion 116 of thebore 114, thus providing a restricted path to or from the pressureeifective area 130 at the end of valve 120. The intersection of bore 114and bore 104 is indicated at 132 and it can thus be seen that springchamber 115 of bore 114 is in communication with the restricted portion108 of passage 104.

j A stepped bore 134 is the last of the three bores intersecting bore104 and includes an enlarged portion 136 extending to the exterior ofhead portion 20, and having screw threads 138 therein, and a reducedportion 140 the sprin communication between the pressure chamber 68 andthe intersecting bore 104 in its constricted portion and extending tothe pressure chamber 68. Bore 134 is closed by a: fluidsealing'plug 1 39which engagesthe threads 138% Bore; 134 contains a valve 142 having ashoulder 144 adapted to engage the shoulder 146 formed at the stepbetweenthe enlarged portion 136 and the reduced portion 140. A. spring147 resiliently biases the valve 142 to a. normal position determinedbyythe engagement of these exerted against the nose 150 of valve. 142. Adashpot action is provided for the valve 1.42 by maintaining arelatively close fit between the periphery of shoulder 144 and bore 136.I

A stepped delivery passage-152 extends. from the pressure chamber 68 tothe exterior of the head portion 20 terminating in a threaded connectionport 154. At the point of entry to pressure chamber 68 passage 152includes. a short portion of a reduced cross-section to provide anorifice 155 whose diameter can be held to close tolerances if' desired.Delivery passage 152 intersects bore 114 containing relief. valve-120 ascan best be seen in Figure 7 Fluid pressure in passage 152 is thusexerted on. the pressure effective area 130 of the relief valve 120aspreviously described. p An obli uely drilled passage 156 in the headportion 20 extends from. the enlarged portion 136 of the bore 134 tointersect passage 152 as can be seen by'reference to Figures 1, 2, and7.. Passage: 1-56 intersects passage 152 between the orifice 155 and theintersection of passage 152 with relief valve bore 114. It is thereforeapparent valve 142 is subjected to two opposing. pressure forces inaddition to the force exerted by the spring 147. The first of thesepressure forces is that due to the pressure existing in pressure chamber68 exerted on the nose 150 of the valve 142. The second of thesepressure forces is that existing in the passage 152, downstream from theorifice 155, which is communicated through the drilled passage 156 toact. on the opposite endof valve 142. As flow in. passage. 152increases, a point will be reached where the pressure drop acrossorifice 154 is. sufiiciently great to permit the pressure force actingon the nose 150 of the valve 142 to shift. the valve against the forceof spring 147 and the pressure force aiding The resulting shift of valve142 establishes bore 104.

Head portion 20 also includes a passage 158 extending from the. flange-102 to the spring chamber of. the relief fold block. 160 is adapted tobe secured v to the pump body 14 by socket. head screws 162 which extendtherefrom into tapped holesin the flanges 92 and 102.. Gaskets 163 and165 insure a fluid tight seal. between the faces 98 and 100 of theflanges, and the manifold block.

A generally U' shaped passage 164 in manifold block 160 includes aninlet 166 and an outlet 168 which are so positioned as to be alignedwith bore 104 and passage '94 respectively. Manifold block also has apairof connection ports 170 and. 172 therein. Port 170 is in fluidcommunication with the U shaped passage 164 while port 172 is in fluidcommunication with a drilled passage 174 in manifold 160 which is sopositioned as to be aligned. with passage 158 in the head portion 20 ofthe housing 14. Gaskets 163 and 165 are provided with suitable .holesfor screws 162 to pass through and to permit fluid communication betweenthe aligning passages in housing 14 and the manifold block 160..

Operation of the unit can best be understood by reference to- Figure 9,which is a schematic arrangement of the units components and passagesand in which a typical application is shown. In the application shown, asteering. booster 176, which may be of the type described in the patentto Vickers 2,022,698, is fixed to the frame of the motor vehicle at 178,connected to the steering linkage by a rod and controlled by a pitmanarm 182. A fluid delivery line 184' connects the pump outlet port 154 tothe booster inletport' 186 while a fluid return line- 188 connects thebooster outlet port 190 to the return port 112. A tank 192, vented toatmosphere at 194 is connected into the system by two fluid conduits 196and 198. Conduit 196 connects: the interior of tank 192 to the U shapedpassage 164 and conduit 198 connects the tank interior to passage 158;It should be noted that, while the conduit connections shown areschematically correct, in an actual installation conduits 196- and 198would be connected to ports 170 and 172, respectively, in the manifoldblock 160. In operation, shaft 34' is driven by the engine of thevehicle by any suitable means. Fluid is taken into the suction .zones'from the inlet kidney ports 72 and 74 and. is discharged from thepumping zones 62 into the kidney ports 76 and 78 in a manner well knownin the v art. Pressure fluid discharged into kidney ports 76 and 78passes through pressure plate 50- and into pressure" chamber 68. Duringrelatively low speed operation of the pump, fluid will pass frompressure chamber 68 to the delivery passage 1-52 at. a rate equal to thedischarge rate of the pumping mechanism. This fluid will then passthroughv the conduit 1-84 to the inlet port 186 of: the steering booster176. Returnv fluid from the booster 176 will pass out port 190 andreturn through conduit 188 to the passage 110 in the pump body 14; Frompassage 110, the returning fluid willv enter the enlarged: portion 106of the bore 104 passing from there into the U- shaped passage 164,thence to the. passage 94in the body portion 16- and return through thebranched. passage 92 to the inlet kidneyports 72 and 74-.

However, due to unavoidable leakage and also the fact that booster unit176 is of. the differential piston type thus taking inmore fluid, whilemoving in a particular direction, than is being expelled, the quantityof returning fluid may at. times be less than that being: deliveredthrough. delivery passage 152. To prevent cavitation and its consequent.noise, it is necessary that this quantitative discrepancy be made upfrom the supply ofi fluidin tank 192. At the lower pump speeds theatmospheric pressure exerted: on the fluid in tank 192 is adequate tocause flow from the tank 192 through the conduit 196 into U' shapedpassage 164 and thence through passages 94 and 92 and kidney ports 72-and 74 into the suctionzones' 64';

Should the pressure in passage 152 rise to an excessive value asdetermined by spring, 121,. the relief valve 120 will open, permitting,the flow of fluid from passage 152 past the valve seat 118 into thespring chamber 115 of the relief valve. As previously mentioned, the:relief valve spring chamber is in communication with bore 104 and asuflicient quantity of fluid will thus be diverted from delivery passage152 to keep the operating pressure within safe limits. Since the pumpingunit is of suflicient size to satisfactorily operate steering, booster176 while the vehicle engine is running at idling; speed, a quantity offluid will be supplied at higher speed'swhich is greatly in excess ofactual requirements. As previously mentioned, valve 142 normally closesthat portion of the bore 134 extending between pressure chamber 68 andbore 104 but becomes operative upon sufficient. pressure drop existingacross the orifice 155 to shift to the left and thus place pressurechamber 68 in communication with bore -4. As the delivery rate of thepumping unit increases, therefore, a continually increasing amount offluid will be by-passed from pressure chamber 68 to bore 104. Also, asthe speed, and consequently the delivery rate, of the pumping unitincreases, it be comes desirable that replenishing fluid be positivelyintroduced to the pump return passages leading to the suction zones 64to prevent cavitation noises since, as previously noted, replenishing inthe manner described for low speed operation is inadequate at higherspeeds.

The requisite high speed replenishing is obtained by the use of thefluid by-passed from pressure chamber 63 to bore 104 in the followingmanner. Due to the constricted portion 108 in the bore 104, theby-passed fluid attains a relatively high velocity. In accordance withBernoullis equation the high velocity of the oil being by-passed throughthe restricted portion 108 is accompanied by a relatively low staticpressure. As previously noted, the spring chamber of the relief valve120 is in communication with the fluid in the tank 12 through passage158 and conduit 198. Fluid necessary to replace leakage losses will thusbe added to the working fluid at the intersection of the spring chamber115 and the constricted portion 108 of the passage 104 which isschematically indicated by passage 200 of Figure 9. As this highvelocity oil passes from the constricted portion 108 into the relativelylarger areas of the balance of the passages leading to the suction zones64, a part of the kinetic energy which it possesses is converted tostatic pressure which is effective to maintain the passages leading tosuction zones 64 full of fluid and so prevent cavitation. Replenishingfluid from tank 192 is thus positively impelled into the returnpassages.

It is also desirable that a certain portion of the fluid beingcirculated by the pumping mechanism pass through the tank 192 forcooling and the removal of entrained air therefrom. Flow into tank 192from U shaped passage 164, of the fluid to be cooled and deaerated, isinduced through the conduit 196 by the static head derived from theconverted kinetic energy of the by-passed fluid. The fluid so withdrawnfrom the system is replaced by fresh fluid from the tank 192 passingthrough the passage 158, valve spring chamber 115 and bore 104 in thesame manner as the leakage replenishing fluid, thus establishing a bleedloop.

Conduit 196 from the reservoir 192 to the U shaped passage 164 maintainsconstant communication between the pump return passage and thereservoir. The role conduit 196 plays in the circuit, however, is achanging one dependent on the speed at which pump 10 is driven. A slowpump speeds, conduit 196 serves to maintain the pump inlet passages fullof fluid and under a pressure determined by atmospheric pressure plusthe liquid head due to the height of the* tank. Fluid necessary toreplenish leakage losses in the system will pass from tank 192 topassage 164 via conduit 196 and initial filling can be achieved bymerely adding fluid to the tank. It can thus be seen that desirablecharacteristics of the open circuit have been retained. As the pumpsspeed increases to the point where cavitation normally becomes aproblem, valve 142 becomes operative to permit fluid to pass frompressure chamber 68 into the restricted portion 108 of the bore 104,thus increasing the pressure on suction zones 64, as previouslydescribed, to prevent cavitation. This pressure increase also inducesflow from passage 164 to tank 192 through conduit 196, thus makingconduit 196 a link in the bleed loop previously discussed.

There is thus provided a compact, efficient, and low '8 cost power packunit particularly well adapted for use with a variable speed primemover. Output to any driven device is maintained at a relativelyconstant rate by the use of a by-pass circuit controlled by a valveresponsive to flow in the pump delivery line.

Advantages of open circuit operation at low speeds have been retainedand cavitation is prevented by increasing the pump inlet pressure athigher speeds. This increase is achieved by utilization of the flow inthe bypass circuit. Since the by-pass circuit becomes operative only atthe higher pump speeds, it can be seen that a system has been providedwhich makes a timely change from effectively open circuit operation toclosed circuit operation without the use of any special valve or otherspecial moving part. Further, during closed circuit operation,circulation of a portion of the working fluid into the reservoir ismaintained and its replacement by fresh fluid from the reservoir hasbeen provided.

While the form of embodiment of the invention as herein disclosedconstitutes a preferred form, it is to be understood that other formsmight be adopted, all coming within the scope of the claims whichfollow.

What is claimed is as follows:

1. In a fluid pressure generating device; pumping mechanism havingsuction and delivery zones; a housing for said pumping mechanism havingtherein external delivery and return connection ports; means in saidhousing forming a delivery passage connecting said delivery port to saiddelivery zones; a reentrant return passage comprising means forming afirst-fluid passage in said housing connected to said return connectionport and having an outlet from said housing, means forming a secondfluid passage in said housing having an inlet to said housing andleading to said suction zones, and a manifold block secured to saidhousing and having therein means forming a third fluid passageinterconnecting the outlet of said first fluid passage and the inlet ofsaid second fluid passage; and an external tank connection port in saidmanifold block leading to said third fluid passage.

2. In a fiuid pressure generating device: pumping mechanism havingsuction and delivery zones; a housing for said pumping mechanism havingtherein external delivery and return connection ports; means in saidhousing forming a delivery passage connecting said delivery port to saiddelivery zones; a reentrant return passage comprising means forming afirst-fluid passage in said housing connected to said return connectionport and having an outlet from said housing, means forming a secondfluid passage in said housing having an inlet to said housing andleading to said suction zones, and a manifold block secured to saidhousing and having therein means forming a third fluid passageinterconnecting the outlet of said first fluid passage and the inlet ofsaid second fluid passage; an external tank connection port in saidmanifold block leading to said third fluid passage; means in saidhousing forming a by-pass passage between said delivery zone and saidreentrant return passage, said by-pass passage being restricted, atleast in part, relative to said reentrant return passage; valve meansresponsive to flow in said'delivery passage to control flow in saidby-pass passage; a second external tank connection port in said manifoldblock; and means forming a fluid passage connecting said second externaltank connection port to the relatively restricted portion of saidby-pass passage, comprising passage means in said housing and passagemeans in said manifold block coincident at the juncture of said housingand said manifold block.

3. The combination of: a tank; a pump comprising a housing havingpumping mechanism therein and external delivery and return connectionports in said housing; means in said housing forming a delivery passageconnecting said delivery port to said delivery zone; a reentrant returnpassage comprising means forming a firstfluid passage in said housingconnected to said return connection port and havingan outlet from saidhousing,

connecting the outlet of said first fluid passage and the inlet of saidsecond fluid passage; a first external tank connection port in saidmanifold block leading to said third fluid passage; means in saidhousing forming a by-pass passage between said delivery zone and saidreentrant return passage, said by-pass passage being restricted, atleast in part, relative to said reentrant return passage; valve meansresponsive to flow in said delivery passage to control flow in saidby-pass passage; a second external tank connection port in said manifoldblock; means forming a fluid passage connecting said second externaltank connection port to the relatively restricted portion of said bypasspassage, comprising passage means in said housing and passage means insaid manifold block coincident at the juncture of said housing and saidmanifold block; a

pair of external connection ports in said tank; two fluid conduitsinterconnecting said tank and said manifold block, one of the conduitsleading from one of the external connection ports in the tank to thefirst external tank connection port in said manifold block, and theother of the two conduits leading from the other of the pair of externalconnection ports in said tank to the second external tank connectionport in said manifold block, thereby establishing a bleed-loop throughthe tank.

4. In combination with pumping mechanism having suction and deliveryzones; means forming a delivery passage leading from said delivery zone;means forming a return passage leading to said suction zone; meansforming a by-pass passage between said delivery zone and said suction-zone, said means including at least part of said return passage andhaving therein a portion constricted relative to that part of saidby-pass passage located downstream from said constricted portion; valvemeans responsive to flow in said delivery passage to control flow insaid by-pass passage; a tank remotely located from said pumpingmechanism; fluid conduit means interconnecting said tank and theconstricted portion of said bypass passage; and fluid conduit meansinterconnecting,

said tank and that portion of the by-pass passage downstream from saidconstricted portion.

5. In a fluid pressure generating device; pumping mechanismhavingfsuctio n and delivery Zones; a housing for said pumping mechanismhaving therein external delivery and return connection ports; means insaid housing forming a delivery passage connecting said delivery port tosaid delivery zones; a reentrant return passage comprising means forminga first fluid passage in said housing connected to said returnconnection port and having an outlet from said housing, means forming asecond fluid passage in said housing having an inlet to said housing andleading to said suction zones, and a manifold block secured to saidhousing and having therein means forming a third fluid passageinterconnecting the outlet of said first fluid passage and the inletofsaid second fluid passage; an external tank connection port in saidmanifold block leading to said third fluid passage; means in saidhousing forming a bypass passage between said delivery zone and saidreentrant return passage, said bypass passage being restricted, at leastin part, relative to said reentrant return passage; valve meansresponsive to flow in said delivery passage to control flow in saidbypass passage; and means, forming a fluid passage having a tankconnection port therein and communicating with the relatively restrictedportion of the bypass passage.

References Cited in the file of this patent UNITED STATES PATENTS2,219,488 Parker Oct. 29, 1940 2,280,392 Herman et al. Apr. 21, 19422,316,445 Marshall Apr. 13, 1943 2,409,975 r Curtis Oct. 22, 19462,437,791 Roth et al Mar. 16, 1948 2,510,150 Stephens June 6, 1950

